The ability of B lymphocytes to capture, process and present antigens to T cells is requisite for normal humoral immune responses and contributes to the pathogenesis of both B and T cell mediated autoimmune diseases. B lymphocytes preferentially capture polyvalent antigens which, by aggregating the B cell antigen receptor (BCR) and initiating signaling cascades, elicit a coordinated series of cellular responses that ensure that even low affinity antigens are productively captured. Antigenic polyvalency greatly accelerates both the endocytosis of engaged receptors and their transit through the endocytic pathway to the late endosomal antigen processing compartments. Similar to what has been described in maturing dendritic cells, BCR ligation also induces a remodeling of the receptor targeted antigen processing compartments which enhances their ability to process peptides and load them onto MHC class II. Despite the importance of BCR signaling in determining antigen presentation to T cells, relatively little is known about which signaling pathways contribute to receptor trafficking and what specific cellular processes they regulate. In this grant application, we demonstrate that the BCR constitutent Igp is ubiquitinylated at the cell surface by the E3 ligase Itch and that this is required for normal trafficking from early to late endosomes. In the absence of ubiquitinylation, the receptor recycles back to the cell surface. Farther downstream, the ubiquitin ligase Cbl-b is required for entry into the antigen processing compartments. From these observations, we propose a model in which there are two different checkpoints in BCR endocytic trafficking each controlled by different ubiquitin ligases. Based on this model, we predict that decisions in receptor trafficking made at each checkpoint determine peripheral B cell responses. We propose to test this model in the following Specific Aims: Aim 1. To determine how Itch ubiquitinylates Igp. Aim 2. To determine the in vivo function of Igp ubiquitinylation. Aim 3. To determine how BLNK and Cbl-b contribute to receptor trafficking. Aim 4: To determine why BCR trafficking is aberrant in anergic B cells.